My 11 year old persian cat cannot walk properly on his back legs?

In geratric cats, common signs of hindlimb weakness are heart disease (hypertrophic cardiomyopathy), noninsulin dependent diabetes mellitus (type 2 diabetes) and chronic renal failure. Unfortunately, if the best vet in the country doesn't have the diagnostic capabilities to screen for these diseases then there isn't much else that you can do. I don't know what type of medications are available in Sri Lanka but my opinion would be to get a second opinion because both DM and HCM , and to a lesser extent CRF, are conditions worth treating.